Fluorescent tube lamp are being more and more replaced by retrofit tube lamps employing semiconductor light emitting elements (light emitting diodes, LEDs). Luminaires can drive the fluorescent tube lamp using either an electronic control gear (ECG) or a conventional control gear (CCG). Retrofit tube lamps for replacing fluorescent tube lamps in a luminaire with CCG require a driver in the form of a switched-mode power supply (SMPS) to provide the LEDs in the lamp with the required operating parameters (voltage, current, power).
The physical size of such a SMPS driver can be rather big due to various application requirements (high power factor, big input filters, wireless functionality, etc.). Therefore, the driver may be split in two parts which are arranged in the two endcaps of a retrofit tube lamp. The two driver parts are then electrically connected, for example by way of traces on the printed circuit board (PCB) which also carries the LEDs. The PCB carrying the LEDs is often referred to as light engine. The output of the driver is fed to the light engine (and, thus, to the LEDs), for example in the middle of the light engine.
The length of the traces connecting the two driver parts (depending on the size of the retrofit lamp, this may be more than 1 m) lead to stray inductances that are formed along the traces. In case that a metal element is present near the traces along their length, for example the lamp housing, a heatsink, a ground metal plate, etc.), stray capacitances may be formed as well.
The SMPS driver for LED retrofit lamps is usually operated in boundary conduction mode, i.e. the current in certain portions of the driver may quickly switch between a maximum value and approximately zero. If the splitting of the SMPS driver into two parts occurs at such a portion of the driver, the high current peaks travelling along a PCB trace connecting the two driver parts may interact with the stray inductances causing voltage fluctuations along the PCB trace.
The voltage fluctuations may interact with the stray capacitances and cause stray currents through the stray capacitances that end up in the metal element (e.g., ground metal plate). These voltage fluctuations and stray currents may increase the electromagnetic interference (EMI) generated by the lamp, in particular in the frequency range of 9 kHz to 30 MHz (conducted EMI), but also in the frequency range of 30 MHz to 300 MHz (emitted EMI). This can make it difficult for the retrofit lamp to stay within statutory and regulatory limits applicable for EMI.
It is, therefore, an object of the present invention to provide LED retrofit lamps which overcome the problems discussed above.